RECEPTOR ORGANS 



the arrangement, in addition to that usually given, namely, the formation of the 

 nerve layer by invagination of the front hemisphere of a spherical outgrowth. 



From the work of von Uexkiill (1912, p. 329) it appears that, whatever 

 image may be formed on the retina of Pecten, no response is called forth until 

 the object moves. Further, the movement of any object excites the same response, 

 which is a protrusion of the long tentacles, endowed with chemical and tactile 

 sensibility. The object of this response is obviously to obtain further information, 

 and flight results if it turns out that the object is an enemy. Otherwise, flight 

 would be a waste of energy. 



When dioptric apparatus is present of sufficient accuracy to form a clear 

 picture on the retina, some mechanism is obviously necessary to adjust the focus 

 for near or distant objects. In land animals, the chief refracting surface is the 

 curved cornea, since the refractive index of the aqueous humour on the inner 

 side of it differs 

 more from that 

 of air than that 

 of the lens does 

 from those of the 

 liquids in which 

 it is immersed. 



This can readily 

 be shown by obser- 

 vation on the eye of 

 an albino rabbit. 

 Owing to the ab- 

 sence of pigment, 

 the image of a dis- 

 tant window with 

 cross-bars can easily 

 be seen through the 

 outer, sclerotic, 

 coat of the eye-ball. 

 If a microscope 

 slide be held in such 

 a position as nearly 

 to touch the cornea, 

 and a drop of phy- 

 siological saline 

 solution be placed 

 between the cornea 

 and the glass, the 

 image disappears, 

 since the refracting 

 surface is now a 

 plane one. On re- 

 moving the glass, 

 the image reap- 

 pears. 



In fish, there- 

 fore, the lens has 



to do the chief work in the formation of an image. Accordingly, we find that 

 its curvature is very much greater than in land animals, the lens being nearly 

 spherical in shape. 



In land animals, the chief use of the lens is to adjust the focus of the dioptric 

 system. The curvature of the cornea is not made to change. According to the 

 work of Beer (1898-1901) accommodation to near or distant objects is effected in 

 two ways. The first is that present in invertebrates, in vertebrates up to and in- 

 clusive of amphibia, together with certain snakes, and consists in the actual change 

 of position of the lens, just as in the photographic camera. The second mechanism 

 is found in a few snakes, in tortoises, lizards, crocodiles, birds and mammals, and 

 consists in a change of the curvature of the lens. In its natural position in the 

 eye, the lens, which is elastic, is focussed for distant objects, owing to the way in 

 which it is pulled upon by the ligaments holding it in place, which cause its 

 curvature to be a flatter one than that assumed when released from tension. But, 



FIG. 161. THE RETINA OF THK DOG. Prepared by Golgi's 

 method. In section. 



a, Cone fibres. 



b, " Granules " and fibres of rods. 



e, d, Bipolar cells (inner granules) with vertical ramifications of their outer processes. 



In the centres of the ramifications lie the enlarged ends of rod fibres. 

 e, Other bipolar cells with flattened ramifications abutting against ramified 'ends of 



cone fibres. 



/, Giant bipolar cell, with flat ramification. 

 <j, Inner granule cell sending axone towards rod and cone fibres. 

 A, Amacrine cell in inner molecular Ia3'er, with diffuse arborisation on ganglion cells. 

 i, Ascending nerve fibre. 

 j, Centrifugal fibres. 



m, Nerve fibres which become lost in inner plexiform layer. 

 i, Ganglion cells which form synapses with the end branches of a bipolar cell belonging 



to a group of rods. 



(Ramon y Cajal, 1894, Taf. V., Fig. 2.) 



